The Tennessee Valley Authority (TVA) delivers electricity to about 10 million residents in parts of seven states, and part of this mission is carried out by 24 units at eight combined-cycle plants that need continuous condition monitoring to improve performance.
To update, improve and expand its condition monitoring, TVA sought to implement Etapro performance monitoring and predictive maintenance software about 18 months ago to create physical models that could continuously monitor heat rates and steam flows to increase overall operating efficiencies, improve reliability, recognize abnormalities and reduce downtime, according to Susan Hobbie, PE, senior I&C engineer, Merrick & Co., an engineering services firm that collaborated with TVA on the project. However, the cost and complexity of bringing in all these parameters quickly turned into an obstacle.
"We originally considered using wired 4-20 mA, serial, analog HART or Foundation fieldbus, but the quantity of instruments involved and networking costs quickly increased to $2-3 million. To minimize these expenses, we settled on using WirelessHART to monitor the 40-60 new instruments we added at each plant," said Hobbie, who presented "TVA uses wireless as backbone for performance monitoring" at the 2019 Emerson Global Users Exchange in Nashville, Tennessee. "These plants also had all different layouts, with two or three units each and control rooms in different locations, so that was another challenge in designing wireless solutions for them."
Hobbie reported that digital modernization with WirelessHART would let it serve as the combined-cycle plants' measurement backbone, and create the physical models that TVA wanted to measure applications, including wireless heat recovery steam generator (HRSG) monitoring, wireless circulation water monitoring, wireless leak valve detection, and wireless weather monitoring.
"We estimate that wireless saved more than $580,000 over the eight plants, which was 15-20% of the total project's material and construction costs, No wire or conduit was needed, minimal downtime was required, and there were no new or extended outages," added Hobbie. "These savings were achieved even though the different control room locations and other unique features required custom WirelessHART designs for each facility.
"Even though we used our existing DCS infrastructure wherever possible, we realized we couldn't design all of these wireless networks ourselves. It was a big help that Emerson came to every site and helped us select the right antennas, such determining when extended-range was needed."
Major wireless players
John Hillencamp, senior sales engineer, Emerson, added that the wireless networks at TVA's eight combined-cycle plants are using Emerson's 1410 gateways with antennas that can be 600 feet away from the transmitters and devices trying connect with them to deliver their data.
"There's a lot of steel in these plants, so it can sometimes be hard to get a signal from a HRSG transmitter to the gateway, but we can use regular or extended-range antennas as needed," said Hillencamp. "We didn't really have ay communication problems at TVA's eight plants because the software they're using only needs to be updated about once per minute. This also enables their batteries to last 8-10 years. We also used the AMS Wireless Snap-On application for managing the wireless networks."
Hillencamp reported that one of the most prevalent components employed at TVA's combined-cycle facilities was Emerson's Wireless 248 temperature transmitter. The 248s worked well for steam drain valves because they have a magnetic temperature sensor that can determine valve position and detect drain valve leaks. Many of these transmitters were installed with Type K thermocouple probes. "They were too costly to put on each transmitter, but they went on 20-30 prioritized valves per plant," added Hillencamp. "Some of these valves were losing $1,782 per month, but using these sensors could pay back their cost twice in one month, so TVA is planning to use more of them."
Similarly, HRGC monitoring was needed because flue gas was fouling, corroding and plugging its tube bundles. Rosemount 3051 S CD2 pressure transmitters were used on the HRSGs because they could maintain designed flow paths, preserve HRGC efficiency, and detect plugged impulse lines.
To monitor ambient weather conditions, Hillencamp added that TVA also implemented Rotronic temperature and humidity sensors along with its Wireless 248 transmitters. "This lets operators know when to chill inlet air a bit more, which can help make the HRGCs more efficient," he added.
"By implementing wireless, TVA now has a digital infrastructure that can be expanded with minimal cost, and with just a 1.5% increase in efficiency at one unit, we could save millions of dollars, and achieve project payback in one year." Merrick and Co.’s Susan Hobbie discussed a money-saving wireless infrastructure project at the Tennessee Valley Authority.
Continuous is comprehensive
Beyond implementing tailored wireless designs at TVA's combined-cycle plants, Hobbie and Hillencamp reported they and their colleagues also needed to account for the different DCSs at each facility, so they could achieve continuous monitoring.
"The plants had standardized on Emerson's Ovation, but some also had DeltaV or a DCS from Siemens or ABB," said Hillencamp. "However, the flexible gateways and protocols we chose could be used with Ovation, DeltaV, Modbus and AMS at all eight plants."
Hobbie reported this flexibility was important becaubse it helped TVA achieve its goal of adopting continuous monitoring instead of periodic monitoring. "Periodic field testing only provides a snapshot of a specific moment in time, which means users have to make assumptions," explained Hobbie. "By monitoring real-time continuous data, we can improve reliability, recognize abnormalities and decrease downtime."
Hillencamp added, "This is the first big wireless project for TVA, so its methods and benefits are likely going to spread to other areas, such as its fossil plants."
Hobbie concluded, "TVA can now create physical performance models as ambient conditions change, which will affect how much steam and power can be produced. By implementing these wireless technologies, TVA now has a digital infrastructure that can be expanded with minimal cost. And, with just a 1.5% increase in efficiency at one unit, we could save millions of dollars, and achieve project payback in one year."
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